Process and plant for cryogenic separation of air

ABSTRACT

A process for cryogenic separation of air using an air separation plant configured as a high air pressure air separation plant is provided, wherein of the air initially compressed several partial air streams are formed which are at least in part further compressed, cooled in a main heat exchanger and expanded before being introduced into the column system. The partial air streams include a first partial air stream and a second partial air stream whose air is compressed in parallel in a first warm booster and a second warm booster and thereafter expanded in a first expander and a second expander mechanically coupled to the first booster and to the second booster, respectively. A corresponding air separation plant is also part of the present invention.

The invention relates to a process for the cryogenic separation of air and to a corresponding plant according to the preambles of the independent patent claims.

BACKGROUND OF THE INVENTION

The production of air products in liquid or gaseous state by cryogenic separation of air in air separation plants (air separation units) is well known and described, for example, in H.-W. Häring (eds.), Industrial Gases Processing, Wiley-VCH, 2006, especially section 2.2.5, “Cryogenic Rectification”.

Air separation plants of the classical type have column systems which can be designed, for example, as two-column systems, especially double-column systems, but also as three- or multi-column systems. In addition to rectification columns for the recovery of nitrogen and/or oxygen in liquid and/or gaseous state, i.e. rectification columns for separation of nitrogen and oxygen, rectification columns can be provided for the recovery of other air components, in particular of noble gases.

The rectification columns of the column systems just mentioned are operated in different pressure ranges. Known double column systems comprise a so-called pressure column (also called high pressure column, medium pressure column or lower column) and a so-called low pressure column (upper column). The high pressure column is typically operated in a pressure range of 4 to 7 bar, especially at about 5.3 bar, whereas the low pressure column is operated in a pressure range of typically 1 to 2 bar, especially at about 1.4 bar. In certain cases, higher pressures can also be used in both rectification columns. The pressures given here and below are absolute pressures at the top of the respective columns.

For air separation, so-called main air compressor/booster air compressor (MAC-BAC) processes or so-called high air pressure (HAP) processes can be used. Main air compressor/booster air compressor processes are the more conventional ones, while high air pressure processes are increasingly used as alternatives in more recent times. A high air pressure process is used in the context of the present invention.

Main air compressor/booster air compressor processes are characterized by the fact that only a part of the total feed air quantity supplied to the column system is compressed to a pressure in a pressure range which is considerably higher than the pressure range in which the pressure column is typically operated (see above). A further part of the feed air quantity is compressed to a pressure in this pressure range only, or at most to a maximum pressure of 1 to 2 bar above it, and is fed into the pressure column without further expansion. An example of such a process is for example shown in FIG. 2.3A of Häring (see above).

In a high air pressure process, however, the total amount of air supplied to the pressure column, and in particular the total amount of air supplied to the column system as a whole, is compressed to a pressure in a pressure range that is significantly higher than the pressure range at which the pressure column is operated. A corresponding pressure range is for example between 10 and 100 bar. The air fed into the pressure column is therefore expanded in a high-pressure process before being fed into the column. High-pressure processes have been described many times and are known from EP 2 980 514 A1 and EP 2 963 367 A1, for example.

In an apparatus for the separation of air by cryogenic distillation disclosed in U.S. Pat. No. 6,257,020 B1, all of the air is compressed to a medium pressure. One part of the air is compressed to an intermediate pressure and a fraction of this air is compressed to a high pressure. The high-pressure air is divided into at least two fractions and expanded in two turboexpanders, the cooled stream from the warm turboexpander being at least partially recycled into the warm end of the exchanger at a higher pressure. A liquid coming from the air separation apparatus vaporizes in the exchanger.

According to U.S. Pat. No. 5,400,600 A, feed air, all of which is compressed to a first high pressure P1, is partly further compressed to a pressure P2. At intermediate temperatures, a portion of each air stream is expanded in a turbine. One of the turbines can have an output at a pressure P3 between P1 and the medium pressure. The major proportion of the separated oxygen is withdrawn as a liquid from the low pressure column, pumped to the production pressure and vaporized in a heat exchange line by condensation or pseudo-condensation of air at one of the pressures P1, P2 and P3, depending on whether the condensation occurs at subcritical or supercritical pressure.

Air separation plants can be designed differently depending on the air products to be supplied and their required aggregate and pressure conditions. For example, the so-called internal compression is known to provide gaseous pressure products. In this process, a cryogenic liquid is taken from the column system, subjected to a pressurization in liquid state, and converted to the gaseous or supercritical state by heating in the main heat exchanger. In this way, for example, internally compressed gaseous oxygen, internally compressed gaseous nitrogen or internally compressed gaseous argon can be produced. Internal compression offers a number of advantages over external compression, which is also possible as an alternative, and is explained e.g. in Häring (see above), Section 2.2.5.2, “Internal Compression”. The invention also relates to an air separation method including internal compression.

In cases in which more unusual product spectra (i.e. amounts or proportions of air products in liquid or gaseous state at certain pressures) are required, classical air separation processes and plants are sometimes not optimal.

For example, a low pressure oxygen product (at about 3 to 9 bar) in combination with a high amount of liquid products is a slightly unusual product spectrum, particularly if a ratio of a value characterizing the total amount of all liquid products, herein referred to as “liquid nitrogen equivalent”, to said low pressure gaseous oxygen product is about 0.93. As to the terms “liquid product” and “gas product” and the definition of the liquid nitrogen equivalent, reference is made to the explanations below. For such a product spectrum, a high-pressure air separation process or plant comprising a so-called excess air turbine was contemplated. In an excess air turbine, compressed and cooled air is reexpanded without being separated, heated and discharged from the air separation plant or returned to the inlet of the main air compressor. In this way, additional cold can be obtained. An air separation plant with an excess air turbine is illustrated for example in U.S. Pat. No. 3,905,201 A. In a high-pressure process, the air supplied to the excess air turbine is typically formed by air that has been expanded together with the air supplied to the pressure column to a pressure in a corresponding pressure range. This air is heated before and after the expansion in the excess air turbine. In the excess air turbine, typically also pressurized nitrogen from the pressure column is expanded and thereafter treated accordingly together with the excess air.

A high air pressure process with an excess air turbine, however, has some disadvantages, as found according to the present invention. Firstly, the relatively high volumetric process air flow in the “warm” part of the plant (in an illustrative example about 150,000 normalized cubic meters per hour at about 15.8 bar) represents a disadvantage in view of capital expense for piping and the molsieve skid/vessels required for conditioning the feed air. Secondly, a relatively large excess air turbine (for expanding the mixed flow including excess air and pressurized nitrogen from the pressure column is required, likewise increasing capital expense. Thirdly, a product compressor for low-pressure nitrogen from the low pressure column, which has to deal with an amount of about 19,000 normalized cubic meters per hour, has to be provided. This also results in high capital expense, as a four-stage compressor is typically required. Low pressure recycles are usually also not very efficient for high liquid production and also require a relatively high heat exchanger surface.

It is an object of the present invention to propose a process and a plant with which the product requirements mentioned above can be met more efficiently and particularly with lower operating and capital expenses.

DISCLOSURE OF THE INVENTION

Against this background, the present invention proposes a process for the cryogenic separation of air and a corresponding plant comprising the features of the independent claims. Preferred embodiments of the present invention are the subject of the dependent claims and the description that follows.

In the following, before turning to the specific features and advantages of the present invention, some basic principles of the present invention are explained and terms used to describe the invention are defined.

For more information relating to the devices and apparatus used in air separation plants, reference is made to technical literature such as Häring (see above), especially Section 2.2.5.6, “Apparatus”. In the following, some aspects of such devices are explained in more detail for clarification and clearer differentiation.

In air separation processes and plants, multi-stage turbo compressors are used to compress all the air to be separated, such compressors being referred to as “main air compressors” or “main compressors” for short. The mechanical design of turbocompressors is basically known to the expert. In a turbocompressor, the medium to be compressed is compressed by means of turbine blades or impellers, which are arranged on a turbine wheel or directly on a shaft. A turbocompressor forms a structural unit, but in a multi-stage turbocompressor this can comprise several compressor stages. A compressor stage usually comprises a turbine wheel or a corresponding arrangement of turbine blades. All these compressor stages can be driven by a common shaft. However, it may also be intended to drive the compressor stages in groups with different shafts, wherein the shafts may also be connected to each other via gears to rotate at different speeds.

The main air compressor is characterized by the fact that it compresses the entire quantity of air fed into the column system which is separated for the production of air products, i.e. the entire feed air. Accordingly, a “booster air compressor” or “post compressor” can also be provided, in which, however, only a part of the air volume compressed in the main air compressor is brought to an even higher pressure. This can also be a turbo compressor. For the compression of partial air volumes, further turbo compressors are typically provided, also referred to as boosters, but in comparison to the main air compressor or the booster air compressor, such further turbo compressors only compress air to a relatively small extent. A booster air compressor may also be present in a high-pressure process where it compresses a partial air volume starting from a correspondingly higher pressure. A booster is referred to as being a “warm booster” if it is supplied with a feed at a temperature above 0° C., particularly above 10° C. or above 20° C. and up to 50° C.

Air can also be expanded at several points in air separation plants, for which purpose expansion machines in the form of turbo expanders can be used. Turbo expanders can also be coupled to and drive turbo compressors. Turbo expanders are also referred to as “expansion turbines”, or for short “turbines” or “expanders” hereinbelow, these terms being used synonymously. If one or more turbo compressors are driven without externally supplied energy, i.e. via one or more turbo expanders, the term “turbine booster” or “booster turbine” is also used for such an arrangement. In a turbine booster, the turbo expander or expansion turbine and the turbo compressor or booster) are mechanically coupled, wherein the coupling can be such as to result in the same speed (e.g. via a common shaft) or in different speeds (e.g. via an interposed gearbox).

In typical air separation plants, appropriate expansion turbines are available at different locations for cooling and liquefaction of fluid streams. These are in particular so-called Joule-Thomson turbines, Claude turbines, Lachmann turbines and the excess air turbines already mentioned above. For the function and purpose of some of these turbines, reference is made to technical literature as well, for example to F. G. Kerry, Industrial Gas Handbook: Gas Separation and Purification, CRC Press, 2006, especially sections 2.4, “Contemporary Liquefaction Cycles”, 2.6, “Theoretical Analysis of the Claude Cycle” and 3.8.1, “The Lachmann Principle”.

Generally, the term “air product”, in the language used herein, shall refer to a fluid in liquid or gaseous state in which a content of at least one air component (nitrogen, oxygen, noble gas) of atmospheric air is higher than in atmospheric air. An air product may be an essentially pure air component, “essentially pure” meaning a content of at least 90%, 95% or 99%. Such an air product may be herein be referred to by using the name of the main component (“oxygen”, “nitrogen”, etc.) only, even if minor amounts of one or more other components are present in the air product. A “liquid product” is an air product which is withdrawn from the air separation plant in liquid state and not evaporated therein, other than internally compressed air products which are initially produced in liquid state and thereafter evaporated or which are withdrawn from the column system already in gaseous state (“gas products”).

The liquid nitrogen equivalent LIN-E mentioned hereinbefore corresponds to the sum of all liquid nitrogen products LIN, all liquid oxygen products LOX multiplied by a factor of 1.07, and all liquid argon products LAR multiplied by a factor of 0.9, wherein all said values are expressed in normalized cubic meters per hour (Nm³/h). In other words, LIN-E [Nm³/h]=LIN [Nm³/h]+1.07×LOX [Nm³/h]+0.9×LAR [Nm³/h].

From an air separation plant, different air products such as liquid oxygen, liquid nitrogen or liquid argon can be exported in liquid state without evaporation as mentioned, wherein correspondingly formed and exported quantities are referred to here with terms such as “liquid quantity”, “liquid output” or “liquid production”, in contrast to air products evaporated by means of internal compression, whose quantities are referred to below as “internal compression quantity”, etc.

In the following, pressure ranges and temperature ranges are given to characterize pressures and temperatures. This is to express that pressures and temperatures need not be in the form of exact pressure or temperature values. For example, in the rectification columns of an air separation plant, different pressures are also present, but all of them lie within a comprehensive pressure range. Different pressure ranges and temperature ranges can be disjunctive ranges or overlapping ranges. The pressures indicated here in bar are always absolute pressures.

Features and Advantages of the Invention

According to an embodiment of the present invention, no recycling streams which are withdrawn from and thereinafter reintroduced in the same column and no excess air expanded in an excess air turbine are used. For providing gaseous feed air to be introduced into the pressure column (the “first partial air stream” mentioned below), according to an embodiment of the present invention, one (main) expander is provided. As a result, the air to be separated in the column system is provided in a relatively low amount of e.g. about 140,000 normalized cubic meters per hour at a pressure level of e.g. about 21.5 bar, resulting, in an embodiment of the invention, in a relatively compact “warm” part of the plant (comprising the components mentioned above).

Further according to the present invention, a Joule-Thomson stream (the “second partial air” stream mentioned below) which is required for evaporating internally compressed oxygen at a pressure in a range of 3 to 9 bar, e.g. about 5 bar, is supplied from a further expander operated with an outlet pressure of e.g. about 13 bar, which also makes this unit quite compact. An improved distribution of refrigeration is achieved, according to an embodiment the present invention, with so-called self-boostering this further expander using a warm booster, as described in the following, but the further expander may also be coupled with a brake and/or a generator, for example, as provided in a further embodiment of the present invention.

For the evaporation of the internally compressed oxygen product in the main heat exchanger, ideally an air counterstream is required which is condensed under a pressure and in a quantity that allows the evaporation process to be carried out with a minimum possible temperature difference. To obtain liquid products, on the other hand, a certain total cooling power (as the sum of the power of the two turbines, in the present case) is required. The efficiency of the plant does not depend on how large this cooling power is, but on how it is divided between the two turbines, as this influences the Q-T profile in the main heat exchanger.

The amount of air provided in the form of the Joule-Thomson stream expanded in the further expander, i.e. the second partial air stream, is mainly defined by the evaporation of the oxygen product. If a warm booster is not used for increasing the pressure of this stream, as may be the in an embodiment of the present invention wherein this further expander is coupled to a brake and/or a generator, the turbine power can only be influenced by its inlet temperature. If the warm booster is used, in contrast, there is practically a further degree of freedom to optimize the turbine power, since not only the inlet temperature but also the inlet pressure can be changed. With the use of a warm booster in an embodiment of the present invention, it was found to be possible to achieve a significantly higher power for the turbine expanding the second partial air stream and a significantly lower power for the (main) turbine, thus noticeably improving the efficiency of the process. Q-T diagrams (enthalpy/temperature profiles in the main heat exchanger) reveal the difference described: If the expander for the second partial air stream is coupled with a braking device such as a generator, for example, as is the case in an embodiment of the present invention, however, the power of the turbine may be e.g. about 452 kW resulting in a temperature decrease from e.g. about 279 K to 238 K, i.e. by 41 K. In case a warm booster is used in a different embodiment of the present invention, in contrast, the power of the turbine may be e.g. about 743 kW and the temperature decrease may be e.g. about 277K to 209 K, i.e. by 68 K. Overall, there is a lower average temperature difference in the main heat exchanger, corresponding to lower thermodynamic losses.

The use of a cold compressor or cold booster as a braking device for the turbine expanding the second partial air stream has proven to be less advantageous, since the cold required for the production of liquid products, which is expensive in terms of energy required for its production, is lost through cold compression. (In this process an excess cooling capacity is not produced, since a large amount of liquid products is/is to be obtained, e.g. corresponding to a ratio of the liquid nitrogen equivalent to internally compressed gaseous oxygen of more than 0.6).

According to the present invention, there is therefore also no issue with unbalanced Ns-numbers (i.e. specific speeds) of expanders and boosters, because the flow for expander and booster is the same. In a one-stage compressor compressing the whole amount of pressurized nitrogen which is withdrawn from the pressure column, an amount of e.g. about 19,000 normalized cubic meters per hour is compressed downstream of a heating in the main heat exchanger.

In a particularly preferred embodiment, a second operation mode is provided in which, instead of a large quantity of liquid oxygen which is provided in a first operation mode, a comparatively much smaller quantity of oxygen is provided and the amount of internally compressed oxygen is significantly increased in comparison with the first operation mode. Further explanations are given below.

According to the present invention, a process for cryogenic separation of air using an air separation plant comprising a column system with a pressure column operated at a pressure in a first pressure range and a low-pressure column operated at a pressure in a second pressure range below the first pressure range is provided. The present invention therefore particularly relates to an air separation plant or unit at least comprising a known double column system as explained at the outset, but also may be configured differently.

According to the present invention, the column system, and thereof at least the pressure column, is supplied with compressed air, all air being supplied to the column system being compressed to a pressure in a third pressure range at least 5 bar above the first pressure range. In other words, and as mentioned before, this corresponds to a high pressure configuration. Of the air compressed to the first pressure range, several partial air streams are formed which are at least in part further compressed, cooled in a main heat exchanger and expanded before being introduced into the column system, According to the present invention, the partial air streams include a first partial air stream whose air is at least in part, in the order indicated and in a single pass, compressed to a pressure in a fourth pressure range above the third pressure range in a first booster, cooled in the main heat exchanger, expanded in a first expander mechanically coupled to the first booster to a pressure in the first pressure range, i.e. it is self-boostered, and introduced into the pressure column. The first booster is operated as a warm booster, according to the present invention, i.e. at an inlet temperature of more than 0° C.

As mentioned, for providing gaseous feed air to be introduced into the pressure column, one (main) expander is provided, which is the first expander just mentioned. As to the advantages of this feature, including the option to provide a relatively compact “warm” part of the plant, reference is made to the explanations above.

The partial air streams include, according to the present invention, a second partial air stream whose air is at least in part, in the order indicated and in a single pass, supplied at a pressure in a fifth pressure range being above or corresponding to the third pressure range to a second expander, expanded in the second expander to a pressure in a sixth pressure range above the first pressure range and below the fifth pressure range, further expanded to a pressure in the first or second pressure range, and introduced into the column system, particularly in liquid or two phase state after an expansion in a Joule-Thomson valve.

Herein, the expression “in a single pass” is intended to refer to an arrangement wherein no air of the partial air stream referred to (i.e. the first or second partial air stream) is heated in the main heat exchanger or recycled to a position upstream of any compression to the third, fourth or fifth pressure level, again compressed and only thereafter introduced into the column system. Preferably the major part of the first and the second partial stream, respectively, i.e. at least 75%, 80% or 90% or all or essentially all of the air of the first and second partial stream is introduced into the column system, particularly without being recompressed.

Furthermore, the air separation plant is operated without a turbine expanding air of the first and/or the second partial stream into the low pressure column. That is, in the present invention, no “Lachmann” or “turbine” air, which is “blown” into the column system is used, and, as mentioned, no recycle streams of feed air are formed.

In an embodiment of the present invention, at least a part of the air of the second partial air stream is, before being expanded in the second expander, compressed to the pressure in the fifth pressure range in a second booster mechanically coupled to the second expander. That is, in this case, the fifth pressure range is above the third pressure range. The second booster is also operated as a warm booster, according to this embodiment of the invention, i.e. at an inlet temperature of more than 0° C. For the advantages using “self-boostering” this turbine, reference is made to the explanations already given above for such an embodiment.

In an alternative embodiment of the present invention, in which the fifth pressure range corresponds to the third pressure range, and therefore air of the second partial air stream is, before being expanded in the second expander, not compressed in a booster, the second expander may be coupled to a braking device, the braking device particularly being selected from an oil brake and an electric generator or a combination thereof. In a yet further embodiment, a combination of a second booster and a braking device may be used for braking the second expander.

According to the present invention and embodiments thereof, therefore, essentially two partial streams which are ultimately fed into the column system are formed, in contrast to an arrangement such as e.g. disclosed in U.S. Pat. No. 5,400,600 A where a Lachmann stream is formed as well, which is blown into the low pressure column. An arrangement as disclosed in U.S. Pat. No. 5,400,600 A is not compatible with the arrangement provided according to embodiments of the present invention as the specific speeds of the turbine(s) and booster(s) are not within the technically feasible range due to the large differences on the amounts (the amount of turbine air, i.e. of the first partial stream would be excessively large and the amount of Joule Thomson air, i.e. of the second partial stream would be comparatively small). That is, the expert would not have considered to modify an arrangement as proposed in U.S. Pat. No. 5,400,600 A to arrive at an arrangement proposed according to embodiments of the present invention.

The air of the second partial air stream is in this connection at least in part cooled before being expanded to the pressure in the sixth pressure range in a first cooling step and at least in part cooled (and liquefied) after having been expanded to the pressure in the sixth pressure range in a second cooling step, the first and second cooling steps being performed using the main heat exchanger.

Yet further according to the present invention, internally compressed gaseous oxygen is produced and withdrawn from the process or plant at an absolute pressure between 3 and 9 bar, preferably between 4 and 6 bar. Therefore, the present invention is specifically adapted to provide the product profile mentioned at the outset.

The second partial air stream corresponds to a Joule-Thomson stream, as mentioned, which is required for evaporating said internally compressed oxygen. As to the advantages of treating this partial air stream according to the present invention, reference is made to the explanations already given above.

As, according to an embodiment of the present invention, the air of the second partial air stream is at least in part compressed to said pressure in a fifth pressure range before the first cooling step, and particularly as in this case the second expander is mechanically coupled to the second booster, the improved distribution of refrigeration already mentioned above is provided. According to a corresponding embodiment of the invention, there is no issue with unbalanced Ns-numbers of expanders and boosters.

According to an alternative embodiment of the present invention, however, the air of the second partial air stream can also be subjected to the first cooling step without being boostered in a warm booster before.

In embodiments of the present invention, the partial air streams include a third partial air stream whose air is at least in part liquefied in the main heat exchanger, expanded to a pressure in the first or second pressure range and introduced into the column system. This third partial air stream is particularly liquefied in the main heat exchanger, thus providing a further Joule-Thomson stream.

When such a third partial air stream is formed, the air of the third partial air stream is, in a preferred embodiment of the present invention, and at least in a first operation mode, at least in part compressed to the pressure in the fourth pressure range in the first booster and liquefied in the main heat exchanger at this pressure.

However, in such an embodiment, the air of the third partial air stream may, in a second operation mode, also be at least in part liquefied in the main heat exchanger at the pressure in the third pressure range, in which case the air of the third partial air stream is not compressed to the pressure in the fourth pressure range in the first booster but bypassed around the first booster by a suitable valve arrangement.

The second operating mode particularly corresponds to an increased formation of internally compressed oxygen on cost of formation of liquid oxygen. In other words, in the second operating mode at least the 1.1-fold, 1.2-fold, 1.3-fold, 1.4-fold, 1.5-fold, 1.6-fold, 1.7-fold, 1.8-fold, 1.9-fold and up to the 2.5-fold or 3.0-fold amount of internally compressed gaseous oxygen may be withdrawn from the air separation plant as compared to the first operating mode and in the second operating mode at most the 0.0-fold, 0.1-fold, 0.2-fold, 0.3-fold, 0.4-fold or 0.5-fold amount of liquid oxygen may be withdrawn from the air separation plant as compared to the first operating mode.

According to the present invention, the first booster, and the second booster are preferably operated with an inlet temperature in a temperature range above 0° C., particularly above 10 or 20° C. and up to about 50° C.

According to the present invention, as mentioned, internally compressed gaseous oxygen at a pressure in a pressure range from 3 to 9 bar is withdrawn from the process or plant. Furthermore, in connection with this, preferably also liquid products are withdrawn from the air separation process or plant, wherein a ratio of a total amount of all liquid products (expressed as the liquid nitrogen equivalent mentioned before) to a total amount of all gaseous oxygen products or the internally compressed gaseous oxygen is in a range from 0.6 to 1.6, at least in the first mode of operation.

According to the present invention, the first pressure range is particularly from 4 to 7 bar, the second pressure range is from 1 to 2 bar, the third pressure range is from 15 to 28 bar, the fourth pressure range is from 25 to 38 bar, the fifth pressure range is, when not corresponding to the third pressure range, from 20 to 45 bar and the sixth pressure range is from 9 to 21 bar absolute pressure.

In the process according to the present invention, furthermore, from the air compressed to the first pressure range, a relative proportion of 0.6 to 0.8 is provided as the first partial air stream and a relative proportion of 0.15 to 0.3 is provided as the second partial air stream. A relative proportion of 0.05 to 0.15 may be provided as the third partial air stream.

As mentioned, the air of the second partial air stream is preferably at least in part liquefied in the main heat exchanger before being expanded to the pressure in the first pressure range or in the second pressure range and thereafter being introduced into the column system.

In this connection, the air of the first partial air stream is preferably at least in part cooled in the main heat exchanger to a temperature in a temperature range between −132 and −92° C. before being expanded in the first expander and the air of the second partial air stream is preferably at least in part cooled in the first cooling step to a temperature in a temperature range between −30 and 30° C. and in the second cooling step to a temperature in a temperature range between −87 and −47° C.

In the process according to the present invention, gaseous nitrogen withdrawn from the pressure column may particularly be heated in the main heat exchanger and thereafter compressed to a product pressure of e.g. 7 to 12 bar.

An air separation plant comprising a column system with a pressure column adapted to be operated at a pressure in a first pressure range, a low-pressure column adapted to be operated at a pressure in a second pressure range below the first pressure range, a first booster adapted to be operated with an inlet temperature of more than 0° C., a first expander mechanically coupled to the first booster, and a main heat exchanger is also part of the present invention.

The air separation plant comprises, according to the present invention, means adapted to supply to the column system, and thereof at least to the pressure column, compressed air, to compress all air being supplied to the column system to a pressure in a third pressure range at least 5 bar above the first pressure range, to form, of the air compressed to the first pressure range, several partial air streams, and to at least in part further compress, cool in a main heat exchanger and expand before being introduced into the column system said partial air streams,

According to the present invention, the partial air streams include a first partial air stream, and the air separation plant comprises means adapted to subject the air of the first partial air stream at least in part, in the order indicated and in a single pass, to a compression in the first booster to a pressure in a fourth pressure range above the third pressure range, to an expansion in the first expander to a pressure in the first pressure range, to a cooling in the main heat exchanger before said expansion to the pressure in the first pressure range, and to an introduction into the pressure column after said expansion to the pressure in the first pressure range.

In the plant according to the present invention, the partial air streams include a second partial air stream, and the air separation plant comprises means adapted to subject the air of the second partial air stream at least in part, in the order indicated and in a single pass, to an expansion from a pressure in a fifth pressure range being above or corresponding to the third pressure range in the second expander to a pressure in a sixth pressure range between the first and the fifth pressure range, to a further expansion to a pressure in the first or second pressure range and to an introduction into the pressure column, The air separation plant according to the present invention is adapted to cool the air of the second partial air stream at least in part before being expanded to the pressure in the sixth pressure range in a first cooling step and to cool the air of the second partial air stream at least in part after having been expanded to the pressure in the sixth pressure range in a second cooling step, and to perform the first and second cooling steps using the main heat exchanger.

Yet further according to the present invention, means are provided, including an internal compression pump, which are adapted to produce in and withdraw from the process or plant internally compressed gaseous oxygen at an absolute pressure between 3 and 9 bar, preferably between 4 and 6 bar, and the air separation plant is adapted to be operated without a turbine expanding air of the first partial stream and/or air of the second partial stream into the low pressure column.

According to an embodiment of the present invention, a second booster may be provided which is adapted to be operated with an inlet temperature of more than 0° C. and which is mechanically coupled to the second expander. This second booster is particularly adapted to compress at least a part of the air of the second partial air stream to the pressure in the fifth pressure range which is, in this case, above the third.

As to further features and specific advantages of the plant according to the present invention, reference is made to the explanations of the inventive method and its embodiments above. This also applies to a plant according to a particularly preferred embodiment of the present invention which comprises means adapted to perform a corresponding method.

The present invention is further described with reference to the appended drawing illustrating an embodiment of the present invention.

SHORT DESCRIPTION OF THE FIGURES

FIG. 1 illustrates an air separation plant according to a particularly preferred embodiment of the present invention.

FIG. 2 illustrates an air separation plant according to a further particularly preferred embodiment of the present invention.

FIG. 3 illustrates an air separation plant according to a further particularly preferred embodiment of the present invention.

FIG. 4 illustrates an air separation plant according to a further particularly preferred embodiment of the present invention.

Hereinafter, explanations relating to methods and steps thereof shall equally apply to apparatus adapted to carry out such method.

EMBODIMENTS OF THE INVENTION

FIG. 1 illustrates an air separation plant 100 according to a particularly preferred embodiment of the present invention.

Air separation plants of the type shown are described elsewhere, for example in H.-W. Häring (eds.), Industrial Gases Processing, Wiley-VCH, 2006, in particular Section 2.2.5, “Cryogenic Rectification”. For detailed explanations of the structure and function, additional reference is made to the relevant technical literature. An air separation plant for the application of the present invention can be designed in a variety of ways, provided it comprises the features claimed.

The air separation plant 100 shown in FIG. 1 comprises, among other things, a main air compressor 1, an absorber skid 2, a main heat exchanger 3, an expansion turbine 4 coupled to a booster 5, i.e. a booster turbine, an expansion turbine 6 coupled to a booster 7, i.e. a further expansion turbine, internal compression pumps 8.1 and 8.2, a counter-stream subcooler 9 and a column system 10.

In the example shown, the distillation column system 10 comprises a classical double column arrangement consisting of a high-pressure column 11 and a low-pressure column 12 as well as a raw argon column 13 and a pure argon column 14.

In the air separation plant 100, a feed air stream formed from atmospheric air A is aspired and compressed by means of the main air compressor 1 via a filter not individually labelled. The air separation plant is operated on the basis of a high-pressure process, and therefore the air is compressed to a pressure in a correspondingly high pressure range which is referred to as “third pressure range” before. The compressed feed air stream, still indicated A, is optionally pre-cooled in a pre-cooling unit not shown in specific detail and thereafter purified in adsorber skid 2 in a manner known per se.

A partial stream B of the compressed and purified air stream A is further compressed in the booster 5 coupled to the expansion turbine 4 to a pressure in a pressure range above the initial pressure range, which is referred to as a “fourth pressure range” before. The booster 5 and the expansion turbine 4 were referred to as a “first booster” and “first expansion turbine” before, respectively. The booster 5 is operated as a warm booster as defined above.

A partial stream C of the partial stream B, referred to as “first partial air stream” before, is, at the pressure in the fourth pressure range in this specific example, partially cooled in the main heat exchanger 3 before being expanded in the first expansion turbine 4 and introduced into the pressure column 11. Expansion in the first expansion turbine 4 is performed to a pressure in a pressure range at which the pressure column 11 is operated and which is referred to as a “first pressure range” hereinbefore.

Another partial stream D of the partial stream B, referred to as “third partial air stream” before, is, at the fourth pressure range, fully cooled and liquefied in the main heat exchanger 3 before being expanded using a valve not individually labelled to a pressure in the first pressure range and introduced into the pressure column 11. Alternatively, the last expansion step may mentioned be also performed to a pressure in a pressure range at which the low-pressure column 12 is operated, this pressure range being referred to as a “second pressure range” before, in which case the third partial air stream may be introduced into the low pressure column 12.

A further partial stream E of the compressed and purified air stream A, which is referred to as “second partial air stream” before, is, in the example shown here, further compressed in the booster 7 coupled to the expansion turbine 6 to a pressure in a pressure range also being above the initial pressure range, which is referred to as a “fifth pressure range” before, and thereafter cooled and liquefied in the main heat exchanger 3 before being expanded in the expansion turbine 6, reintroduced into the main heat exchanger 3, further cooled therein, and introduced into the pressure column 11. The booster 7 and the expansion turbine 6 were also referred to as “second booster” and “second expansion turbine” before, respectively, and the cooling steps in the main heat exchanger 3 before and after the expansion in the expansion turbine 6 as “first cooling step” and “second cooling step”. The booster 7 is, like the booster 5, operated as a warm booster as defined above. Be it noted that the booster 7 can also be omitted, in non-inventive alternatives, in which case the expansion turbine 6 may also be coupled to an electric generator or a brake instead. In such a case, furthermore, the second partial air stream is provided to the first cooling step at a pressure in the third pressure range.

Expansion in the second expansion turbine 6 is performed to a pressure in a pressure range above the first pressure range and below the third and fifth pressure range, which is referred to as “sixth pressure range” hereinbefore. Thereafter, the second partial stream E is expanded using a valve not individually labelled to a pressure in the first pressure range. Partial streams D and E, i.e. the third and second partial air streams mentioned before, are combined in the example shown before being introduced into the pressure column 11. As for partial air stream D, also the partial air stream E may alternatively be expanded to a pressure in the second pressure range instead of the first pressure range, in which case the partial air stream E may also be introduced into the low pressure column 12.

In the high-pressure column 11, an oxygen-enriched liquid bottom fraction and a nitrogen-enriched gaseous top fraction are formed. The oxygen-enriched liquid bottom fraction is, as known per se, withdrawn from the high-pressure column 11, partly used as heating medium in a sump vaporizer of the pure argon column 14 and fed in defined proportions into a top condenser of the pure argon column 14 and a top condenser of the raw argon column 13. Fluid evaporating in the evaporation chambers of the head condensers of the raw argon column 13 and the pure argon column 14 is combined and transferred to the low-pressure column 12, such as are purge amounts of liquid remaining in these evaporation chambers.

The gaseous nitrogen-rich head product is, on the one hand, withdrawn from the head of the high-pressure column 11, liquefied in a main condenser, which creates a heat-exchanging connection between the high-pressure column 11 and the low-pressure column 12, and refed as a reflux into the high-pressure column 11. A further part is internally compressed in pump 8.1, heated in the main heat exchanger, and provided as an internally compressed gaseous nitrogen product GAN IC. A yet further part is subcooled in the subcooler 9 and expanded into the low pressure column 12. Furthermore, a part or the gaseous nitrogen-rich head product is, in the form of a stream indicated F, heated in the main heat exchanger 3 in gaseous form and in parts provided as seal gas SG and compressed in a compressor 20 for gaseous nitrogen arranged downstream of the warm side of the main heat exchanger.

An oxygen-rich liquid bottom fraction and a nitrogen-rich gaseous top fraction are formed in the low-pressure column 12. The former is partially pressurized in liquid form in the pump 8.2, heated in the main heat exchanger 3, and made available as an internally compressed gaseous oxygen product ICGOX 1. A further part is at least in part subcooled in the subcooler 9 and provided as a liquid oxygen product LOX. A liquid nitrogen-rich stream is withdrawn from a liquid retention device at the head of the low-pressure column 12 and is discharged as liquid nitrogen product LIN from the air separation unit 100. A gaseous nitrogen-rich stream withdrawn from the head of the low pressure column 12 is passed through subcooler 9 and the main heat exchanger 5 and provided as nitrogen product LPGAN at the pressure of the low pressure column 12. Furthermore, a stream is withdrawn from the low-pressure column 12 from an upper section and, after heating in the main heat exchanger 3, is used as so-called impure nitrogen in the pre-cooling unit not shown, or, after heating by means of an electric heater, as a regeneration stream in the adsorption skid 2. It is thereafter vented to the atmosphere ATM.

The operation of the argon system comprising the raw argon column 13 and the pure argon column 14 is generally known and will not be explained in more detail. Using the argon system, a liquid argon product LAR is provided, while mainly nitrogen from the top of the pure argon column may be vented to the atmosphere.

In a specific example, the air stream A may be provided in an amount of about 139,700 normalized cubic meters per hour and at a pressure of about 21.50 bar (third pressure range) while the further compression in the booster 5 is performed to a pressure of about 31.6 bar (fourth pressure range). Partial air stream D, i.e. the first partial air stream, is, in this example, formed in an amount of about 11,000 normalized cubic meters per hour. In the first turbine 4, an amount of work corresponding to about 1,688 kilowatts is provided. In the example, the partial air stream E, i.e. the second partial air stream, may be compressed in the second booster 7 to a pressure of about 37.5 bar (fifth pressure range). Partial air stream E is, in this example, formed in an amount of about 31,750 normalized cubic meters per hour. In the second turbine 6, an amount of work corresponding to about 742 kilowatts is provided. The outlet pressure of the second turbine 6 (fifth pressure range) is, in the example, about 13 bar.

As to the air products provided, in the example just mentioned, pressurized gaseous nitrogen PGAN may be provided in an amount of about 19,200 normalized cubic meters per hour, wherein this pressurized gaseous nitrogen PGAN is supplied to the compressor 20 at a pressure of about 5.1 bar. Internally compressed nitrogen GAN IC is provided in an amount of about 1,160 normalized cubic meters per hour and at a pressure of about 61 bar. The internally compressed oxygen ICGOX 1 is, in the example, provided in an amount of about 18,500 normalized cubic meters per hour and at a pressure of about 4.8 bar. Liquid oxygen LOX is provided in an amount of about 7,600 normalized cubic meters per hour while liquid argon LAR is provided in an amount of about 992 normalized cubic meters per hour. Liquid nitrogen LIN is provided in an amount of about 8,200 normalized cubic meters per hour.

Further fluid streams may be provided and treated as desired, one non-limiting example being illustrated in form of stream X.

FIG. 2 illustrates an air separation plant 200 according to a further particularly preferred embodiment of the present invention.

In contrast to the air separation plant 100 illustrated in FIG. 1 , the first booster 5 may be bypassed in the air separation plant 200 according to FIG. 2 , as illustrated with a stream D.1, which is provided instead of stream D.2, such that air of the partial air stream D may be taken directly from the air stream A, i.e. at the first pressure level. This bypass is realized by closing a valve 202 while opening a valve 201 and preferably realized in an operating mode referred to as “second operating mode” before. By opening valve 202 while closing valve 201 instead, essentially the operation as illustrated in FIG. 1 for air separation plant 100 before is realized by providing stream D.2 and not stream D.1, this operating mode being referred to as a “first operating mode” before.

The second operating mode particularly corresponds to an increased formation of internally compressed oxygen ICGOX 1 on cost of formation of liquid oxygen LOX. For example, production of internally compressed oxygen ICGOX 1 may, in the second operating mode, be increased to an amount of about 26,000 cubic meters per hour while production of liquid oxygen may be reduced to an amount of about 100 normalized cubic meters per hour. (Production amounts in the first operating mode may essentially correspond to those discussed before for air separation plant 100.) As a result of decreased liquid production the total amount of the air supplied in the form of the air stream A is not reduced, stream A being e.g. provided in an amount of about 141,500 normalized cubic meters per hour, but its pressure is reduced to e.g. about 17.4 bar absolute pressure. This means that the first booster 5 might not be able to cope with the significantly increased volume of air supplied to it. Here the bypass D.1 via valve 201 comes into play, reducing the load on the first booster 5.

In said second operating mode, and in the air separation plant 200, the air stream A may be provided in an amount of about 141,500 normalized cubic meters per hour and at a pressure of about 17.4 bar (third pressure range) while the further compression in the first booster 5 is performed to a pressure of about 23.1 bar (fourth pressure range). In the first turbine 4, an amount of work corresponding to about 1,331 kilowatts is provided. In the example, the partial air stream E, i.e. the second partial air stream, may be compressed in the second booster 7 to a pressure of about 24 bar (fifth pressure range). Partial air stream E is, in this example, formed in an amount of about 24,000 normalized cubic meters per hour. In the second turbine 6, an amount of work corresponding to about 344 kilowatts is provided. The outlet pressure of the second turbine (sixth pressure range) is, in the example, about 12.6 bar. The third partial air stream D is formed in an amount of about 22,500 normalized cubic meters per hour.

As to the air products provided, in said second operating mode, and in the air separation plant 200, pressurized gaseous nitrogen PGAN may be provided in an amount of about 19,200 normalized cubic meters per hour, wherein this pressurized gaseous nitrogen PGAN is supplied to the compressor 20 at a pressure of about 5.3 bar. Internally compressed nitrogen GAN IC is provided in an amount of about 1,160 normalized cubic meters per hour and at a pressure of about 61 bar. The internally compressed oxygen ICGOX 1 is, in the example, provided in an amount of about 26.00 normalized cubic meters per hour, as mentioned already, and at a pressure of about 4.8 bar. Liquid oxygen LOX is provided in an amount of about 100 normalized cubic meters per hour, as also mentioned, while liquid argon LAR is provided in an amount of about 992 normalized cubic meters per hour, low pressure gaseous nitrogen LPGAN is provided in an amount of about 10,500 normalized cubic meters per hour, and liquid nitrogen is provided in an amount of 10,200 normalized cubic meters per hour.

As thus can be seen, the second operation mode may be used to provide the other air separation products except internally compressed oxygen ICGOX 1 and liquid oxygen LOX, in essentially the same amounts.

In other words, the air of the third partial air stream (D) is, in a first operation mode of the air separation plant 200 and in the air separation plant 100, at least in part compressed to the pressure in the fourth pressure range in the first booster 5 and liquefied in the main heat exchanger 3 at the pressure in the fourth pressure range. In contrast, the air of the third partial air stream D is, in a second operation mode of the air separation plant 200, but not in the air separation plant 100, at least in part liquefied in the main heat exchanger 3 at the pressure in the third pressure range.

FIG. 3 illustrates an air separation plant 300 according to a further particularly preferred embodiment of the present invention.

In the air separation plant 300, liquid oxygen internally compressed using pump 8.2 is, before being heated in the main heat exchanger 3, split into two partial streams and, after being passed through the main heat exchanger 3, provided in the form of two different fraction of internally compressed oxygen, ICGOX 1 and ICGOX 2.

FIG. 4 illustrates an air separation plant 400 according to a further particularly preferred embodiment of the present invention.

As illustrated in FIG. 4 , the partial stream E of the compressed and purified air stream A, which is referred to as “second partial air stream” herein is not further compressed in a booster such as the booster 7 shown before. That is, the “fifth” pressure range referred to above in this case corresponds to the “third” pressure range. The expansion turbine 6 is, in the example illustrated in FIG. 4 , mechanically coupled to a generator G. 

1. A process for cryogenic separation of air using an air separation plant comprising a column system with a pressure column operated at a pressure in a first pressure range and a low-pressure column operated at a pressure in a second pressure range below the first pressure range, wherein the column system, and thereof at least the pressure column, is supplied with compressed air, all air being supplied to the column system being compressed to a pressure in a third pressure range at least 5 bar above the first pressure range, and of the air compressed to the first pressure range, several partial air streams being formed which are at least in part further compressed, cooled in a main heat exchanger and expanded before being introduced into the column system, the partial air streams include a first partial air stream whose air is, at least in part, in the order indicated and in a single pass, compressed to a pressure in a fourth pressure range above the third pressure range in a first booster being operated with an inlet temperature of more than 0° C., cooled in the main heat exchanger, expanded in a first expander mechanically coupled to the first booster to a pressure in the first pressure range, and introduced into the pressure column, the partial air streams include a second partial air stream whose air is at least in part, in the order indicated and in a single pass, supplied at a pressure in a fifth pressure range being above or corresponding to the third pressure range to a second expander, expanded in the second expander to a pressure in a sixth pressure range between the first and the fifth pressure range, further expanded to a pressure in the first or second pressure range, and introduced into the column system, the air of the second partial air stream is at least in part cooled before being expanded to the pressure in the sixth pressure range in a first cooling step and at least in part cooled after having been expanded to the pressure in the sixth pressure range in a second cooling step, the first and second cooling steps being performed using the main heat exchanger, internally compressed gaseous oxygen is produced in and withdrawn from the process at an absolute pressure between 3 and 9 bar, and the air separation plant is operated without a turbine expanding air of the first partial stream and/or air of the second partial stream into the low pressure column.
 2. The method according to claim 1, wherein the fifth pressure range is above the third pressure range, wherein at least a part of the air of the second partial air stream is, before being expanded in the second expander, compressed to the pressure in the fifth pressure range in a second booster mechanically coupled to the second expander, the second booster being coupled with an inlet temperature of more than 0° C.
 3. The method according to claim 1, wherein the partial air streams include a third partial air stream whose air is at least in part liquefied in the main heat exchanger, expanded to a pressure in the first or second pressure range and introduced into the column system.
 4. The method according to claim 1, wherein the air of the third partial air stream is, at least in a first operation mode, at least in part compressed to the pressure in the fourth pressure range in the first booster and liquefied in the main heat exchanger at the pressure in the fourth pressure range.
 5. The method according to claim 4, wherein the air of the third partial air stream is, in a second operation mode, at least in part liquefied in the main heat exchanger at the pressure in the third pressure range.
 6. The method according to claim 5, wherein, in the second operating mode at least the 1.1-fold and up to the 3.0-fold amount of internally compressed gaseous oxygen is withdrawn from the air separation plant as compared to the first operating mode and wherein, in the second operating mode, at most the 0.5-fold amount of liquid oxygen is withdrawn from the air separation plant as compared to the first operating mode.
 7. The process according to claim 1, wherein liquid products are withdrawn from the air separation plant, wherein a ratio of an equivalent value characterizing a total amount of the liquid products to a total amount of the internally compressed gaseous oxygen is in a range from 0.6 to 1.6, the equivalent value corresponding to the sum of all liquid nitrogen products, all liquid oxygen products multiplied by a factor of 1.08, and all liquid argon products multiplied by a factor of 0.8, all values being expressed in normalized cubic meters per hour.
 8. The process according to claim 1, wherein the first pressure range is from 4 to 7 bar, the second pressure range is from 1 to 2 bar, the third pressure range is from 15 to 28 bar, the fourth pressure range is from 20 to 38 bar, the fifth pressure range is from 20 to 45 bar and the sixth fifth pressure range is from 9 to 21 bar absolute pressure.
 9. The process according to claim 1, wherein, from the air compressed to the third pressure range, a relative proportion of 0.6 to 0.8 is provided as the first partial air stream and a relative proportion of 0.15 to 0.30 is provided as the second partial air stream.
 10. The process according to claim 2 wherein, from the air compressed to the third pressure range, a relative proportion of 0.05 to 0.15 is provided as the third partial air stream.
 11. The process according to claim 1, wherein the air of the second partial air stream is at least in part liquefied in the main heat exchanger before being expanded to the pressure in the first pressure range or in the second pressure range and thereafter being introduced into the column system.
 12. The process according to claim 1, wherein the air of the first partial air stream is at least in part cooled in the main heat exchanger to a temperature in a temperature range between −132 and −92° C. before being expanded in the first expander and wherein the air of the second partial air stream is at least in part cooled in the first cooling step to a temperature in a temperature range between −30 and 30° C. and in the second cooling step to a temperature in a temperature range between −87 and −47° C.
 13. The process according to claim 1, wherein gaseous nitrogen withdrawn from the pressure column is heated in the main heat exchanger and thereafter compressed to a product pressure.
 14. An air separation plant comprising a column system with a pressure column adapted to be operated at a pressure in a first pressure range, a low-pressure column adapted to be operated at a pressure in a second pressure range below the first pressure range, a first booster adapted to be operated with an inlet temperature of more than 0° C., a first expander mechanically coupled to the first booster, a second expander, and a main heat exchanger, wherein the air separation plant comprises means adapted to supply to the column system, and thereof at least to the pressure column, compressed air, to compress all air being supplied to the column system to a pressure in a third pressure range at least 5 bar above the first pressure range, to form, of the air compressed to the pressure in the first pressure range, several partial air streams, and to at least in part further compress, cool in the main heat exchanger and expand before being introduced into the column system said partial air streams, the partial air streams include a first partial air stream, and the air separation plant comprises means adapted to subject the air of the first partial air stream at least in part, in the order indicated and in a single pass, to a compression in the first booster to a pressure in a fourth pressure range above the third pressure range, to an expansion in the first expander to a pressure in the first pressure range, and a cooling in the main heat exchanger before said expansion to the pressure in the first pressure range, and to an introduction into the pressure column after said expansion to the pressure in the first pressure range, the partial air streams include a second partial air stream, and the air separation plant comprises means adapted to subject the air of the second partial air stream at least in part in the order indicated and in a single pass, to an expansion from a pressure in a fifth pressure range being above or corresponding to the third pressure range, in the second expander to a pressure in a sixth pressure range between the first and the fifth pressure range, to a further expansion to a pressure in the first or second pressure range and to an introduction into the column system, the air separation plant is adapted to cool the air of the second partial air stream at least in part before being expanded to the pressure in the sixth pressure range in a first cooling step and to cool the air of the second partial air stream at least in part after having been expanded to the pressure in the sixth pressure range in a second cooling step, and to perform the first and second cooling steps using the main heat exchanger, the air separation plant comprises means which are adapted to produce in and withdraw from the air separation plant internally compressed gaseous oxygen at an absolute pressure between 3 and 9 bar, and the air separation plant is adapted to be operated without a turbine expanding air of the first partial stream and/or air of the second partial stream into the low pressure column.
 15. The air separation plant according to claim 14, wherein a second booster is provided which is adapted to be operated with an inlet temperature of more than 0° C. and which is mechanically coupled to the second expander, wherein the second booster is adapted to compress at least a part of the air of the second partial air stream to the pressure in the fifth pressure, the fifth pressure range being above the third pressure range. 